Certain surgeries, particularly coronary artery bypass surgery, necessarily involve the use of suture needles of small diameter having exceedingly high bending stiffness and strength. In particular, surgeries of this type require that the suture needle's path be closely controlled. In the event that a needle were to flex excessively as it entered the tissue or as it pierced the inner surface of a blood vessel before re-emerging, improper placement of the needle and serious trauma to the tissue and the patient could possibly occur.
In use, suture needles are subjected to forces sufficient to overcome frictional drag through the tissue. These forces, which tend to resist needle penetration, may be greater in patients undergoing cardiovascular surgery who exhibit calcified or toughened tissue due to coronary artery disease. In these procedures, the suture needle must be able to pass through not only the blood vessel, but also any hard calcified tissue that may be located along the periphery of the blood vessel lumen. An overly compliant needle may deflect elastically during tissue penetration and may result in some loss of placement control. As such, it is preferable that the needle should have a relatively high bending stiffness, that is, a low tendency to flex and high tendency to retain its configuration when subjected to a deforming force. Hence, stiffness in bending is a desirable property for the handling and performance of suture needles. A stiff needle resists elastic deflection and can thus be directed as intended to provide a high level of control.
ASTM Standard F1840-98a (Reapproved 2004) provides standard terminology for surgical suture needles and ASTM standard F1874-98 (Reapproved 2004) provides details of a standard test method for bend testing of needles used in surgical sutures. Both ASTM standards are incorporated herein by reference in their entirety. Two different measures for the strength of surgical suture needles are used, namely, yield bend moment, which is the amount of moment required to initiate plastic deformation during a bend test, and maximum bend moment, which is the greatest moment applied to a needle during a bend test. This later value of maximum bend moment is typically measured at a point where the needle has undergone substantial plastic deformation and is generally higher than the yield bend moment or point at which plastic deformation initiates. The point of deflection at which plastic deformation initiates, or more formally according to ASTM standards, the angle at which the yield bend moment occurs, is referred to as the yield bend angle.
The vast majority of suture needles used in the practice of medicine are curved. The curvature of the needle enables exact placement and guidance through the surrounding tissue. Curvatures may be slight, for example equivalent to a one-quarter circle, or may be pronounced for example equal to or in excess of one-half circle. Suture needles are conventionally produced by curving straight wire to the desired degree. However, in the process of mechanically curving the suture needle, residual stresses are produced that may serve to weaken the needle and may make it susceptible to bending or opening against the curvature when stress is imparted in the course of surgery. Indeed, the yield moment required to bend a curved suture needle may be substantially less than the yield moment required to bend a straight needle. Thermal processes for relieving the residual stresses imparted during the needle curving process have been developed to improve strength and stiffness of the suture needle.
Both needle bending strength and needle bending stiffness influence handling characteristics, as well as penetration performance and efficacy of the suture needle. It is important to note that in almost all circumstances, the suture needle should be used in applications where the yield bend moment is not exceeded, since above this value, the needle may bend plastically, may lose its original shape, and may no longer function as intended. It is thus apparent that a desirable characteristic of a suture needle is a high yield bend moment, which is a manifestation of the bending strength of the suture needle. Below the yield bend moment, the resistance to bending of the suture needle is best characterized by the needle bending stiffness.
Needle bending stiffness is a critical measure of the resistance to elastic or recoverable bending of the suture needle before needle deflection reaches the yield bend angle and can be calculated as the yield bend moment divided by the yield bend angle. If a straight or curved suture needle has a low value of bending stiffness, substantial bending of the needle may occur for a given bend moment, whereas if a straight or curved suture needle exhibits a high bending stiffness value, relatively little elastic bending of the needle will occur for a given bend moment. Surgeons tend to perceive a high degree of elastic bending as a loss of control or as poor penetration performance since the needle point is not translating directly with the motion of their hands. As such, needle bending stiffness may be recognized as an important measure of needle performance in most surgical applications.
Hence, desirable bend properties for a suture needle are high bending stiffness, as well as bending strength manifested as high yield bend moment and ductility, in order to penetrate tissue which is being sutured without undue flexing, plastic bending, or breaking during a surgical procedure.
The needle should also not be brittle; if any portion of the needle is too brittle it may break during use if too much force is applied. The needle should instead be ductile, which is the ability to bend without breaking. Curved suture needles are commonly bent through a bend angle of 90 degrees and then manually reshaped to their original curvature to assess ductility. Those skilled in the art of needle making will recognize this procedure as the reshaping process and will further recognize that the higher the number of reshape processes that a needle can withstand without breaking the more ductile it is.
Processes for the thermal treatment of suture needles for the purpose of improving yield moment and stiffness have largely been ignored. This is especially true for stainless steel suture needle materials that do not undergo martensitic transformations such as the 302 SS, 304 SS, 316 SS, 4310 SS, and the like, which employ work hardening, imparted during wire drawing and needle forming operations, as their primary strengthening mechanisms. By way of exception, martensitic and martensitic-aged stainless steels such as 420 SS, 455 SS, 465 SS and others are conventionally subjected to batch heat treatment to drive either a martensitic transformation for strength, precipitation of a strengthening phase, and/or tempering of the alloy to enhance ductility. However, these processes have not been designed expressly for enhanced stiffness and yield moment.
In view thereof, there remains a long-felt need for an apparatus and process for rapidly thermally treating a suture needles for enhanced stiffness and yield moment.